1. Field of the Invention
The present invention generally relates to carrier level balancing circuits in a color camera signal processing circuit and, more particularly, to a carrier level balancing circuit for use in a signal processing circuit of a color camera formed of a CCD (charge-coupled device) solid state image pickup device of a so-called all pixel separately read out type in which signal charges of all pixels are separately read out during one field period.
2. Description of the Prior Art
In a conventional signal processing circuit of a color camera, a color camera formed of a CCD solid state image pickup device of a so-called two-pixel mixing type in which signal charges of two pixels adjacent in the vertical direction are mixed and read out is disclosed in FIG. 12 of Japanese laid-open patent publication No. 63-301687, for example. In this kind of CCD solid state image pickup device, output signals read out by mixing two pixels at every field are respectively multiplied with proper different coefficients thereby to balance carrier levels of the respective output signals. Then, the occurrence of a color aliasing signal in a luminance signal is suppressed by generating a luminance signal from the output signal whose carrier level was balanced. Further, a CCD solid state image pickup device of an all pixel separately read out type is known to improve a vertical resolution as is disclosed in Japanese laid-open patent publication No. 63-301687.
When the carrier levels are balanced according to the aforesaid method, a color aliasing signal occurred when a cameraman takes a picture of an object of an achromatic (black and white) color can be canceled to a certain level. However, since the CCD solid state image pickup device is formed of the two-pixel mixing type, carrier levels are not always balanced due to a difference of sensitivity between pixels of a color filter. Therefore, a color aliasing signal in a luminance signal could not be canceled completely in the vertical and horizontal directions.
On the other hand, in the CCD solid state image pickup device of all pixel separately read out system, if carrier levels are not balanced by any method, then a color aliasing signal will occur in a luminance signal or the like.